Instead, I thought that the resolution is 4 times the pixels compared to the 1280x800 resolution of first Oculus Rift devkit.

Since that current resolution already is "good enough", it means doubling that in both directions will surely move the visual fidelity to "very nice" category.

But surely there are some nice performance implications... :)

Remember that even with the current 1280x800 Rift devkit resolution it's recommended to draw 2170x1360 image for proper image quality (1.7x resolution in both directions). The point is to sample that down to 1280x800 while applying the lens distortion shader.

If we take that 1.7x multiplier and apply it to 2560x1600, we get up to 4352*2720. That's a lot of pixels to fill, so it won't hurt to have as fast as possible desktop GPU.

Just for some comparison, think of the last/current gen console games, which were often set to run at 1280x720 resolution in 30 fps. Since you have to run at 60 fps minimum for a VR HMD, 2560x1600 at 60fps will require 9 times the fill rate at minimum... but, that's not enough, basically you should render at the larger resolution for the supersampling. 4352x2720 at 60fps brings us up to almost 26 times the pixels to fill.

Naturally there are bunch of ways to mitigate the impact, e.g. find ways to skip rendering some of the "lost" pixels at the boundaries which won't be visible in any case.

All that being said, I think it's more likely we'll get the 1080p version first for the consumer targeted device. Oculus already showed that in E3. That should already be a considerable improvement over the devkit resolution. But now we know that it's possible to push the resolution even further.